In a typical powder coating operation, powder particles must be moved from a supply source, such as a hopper, to a powder delivery apparatus, such as a powder spray gun, from which the powder particles are sprayed toward a workpiece to be powder coated. To achieve uniformity in surface coverage on the workpiece, and thereby assure a high quality coating of powder particles, it is necessary to maintain a high level of consistency in powder quantity supplied to the spray gun, even for relatively small units of time. In short, the quality of the powder coating applied to the workpiece depends upon the ability of the entire system to move stable and uniform quantities of powder particles from the hopper to the spray gun, even for small units of time.
In some powder coating systems, the powder particles are maintained in a fluidized state while in the hopper. The fluidizing structure generally includes a gas permeable porous panel at the bottom of the hopper, and compressed gas is blown upwardly through the panel to fluidize the powder particles. This structure is referred to as a fluidized bed. With the powder particles thus maintained in a fluidized state in the hopper, a pneumatic ejector pump in fluid communication with the hopper draws the powder particles out of the hopper and transports them along a conduit to a spray gun or other powder application device.
There are several disadvantages with the use of this type of structure for powder particle supply. For instance, with the fluidized bed hopper/ejector pump combination, it is difficult to maintain the powder particles in a sufficiently fluidized state to continuously transport consistent quantities of powder particles. This is particularly true for powder particles having a relatively small diameter, such as less than ten microns. With powder particles of this size, the powder particles in the hopper generally have very poor overall fluidity and sometimes no fluidity at all. As a result, transport of the particles tends to occur in a pulsed manner. This pulsing produces inconsistencies in the quantity of powder particles transported to the spray apparatus, and subsequently, results in spraying of inconsistent powder quantities.
With powder particles having a relatively large diameter, such as up to several hundred microns, greater volumes of air are needed to maintain the powder particles in a fluidized state in the hopper. However, this increased air volume causes dispersion among the powder particles, resulting in instability, and uniformity of powder particle transport cannot be maintained.
Regardless of particle size, transporting consistent quantities of powder particles becomes increasingly more difficult for powders which have high cohesive properties.
Other powder supply equipment include a screw feeder, an auger feeder or a table feeder. However, with these other types of supply equipment, the same problems also are present, because some form of air transport equipment, such as an ejector pump, must be used in combination therewith.
Additionally, supply devices such as a screw feeder or an auger feeder produce a slight, but inherent pulsing of the supply quantity, due to the cyclical nature of the rotational movement used to perform the supply function. As a result, on the average, supply equipment of this type does produce a stable supply of powder particles over a relatively long period of time, such as several minutes or several hours. However, for very short periods of time, such as several seconds or several milliseconds, this supply equipment causes inconsistencies in the quantities of powder particles supplied.
It is an objective of this invention to improve uniformity and stability in transporting powder particles from a powder particle supply device such as a hopper to a powder spray apparatus, such as a spray gun, even over very small units of time.
It is another objective of this invention to reduce the dispersion of powder particles during transport from a powder particle supply to a powder particle spray apparatus, even over very small units of time, thereby to enhance precision in ultimate application of the powder particles to a surface.
It is still another objective of this invention to achieve uniformity and stability, over relatively small units of time, in the quantities of powder particles transported from a powder supply to a powder spray apparatus, even for powders of different particle size and/or powders having high cohesive properties.